This invention relates generally to a leak sensor apparatus for measuring the leakage in vacuum circuit interrupters and an indicating system adapted to be connected to one or more leak sensors to provide an indication of leakage and instructions for replacing a faulty vacuum circuit interrupter.
The superior characteristics of vacuum as a dielectric make its use in power interrupting devices preferred over the use of special arc extinguishing materials, such as gases and liquids. Since vacuum offers a dielectric strength with a recovery rate of several thousand volts per microsecond, interruption can normally be anticipated at the first current zero in an A.C. current waveform. Furthermore, the total contact stroke required in a vacuum interrupter is a fraction of an inch. The short stroke provides low mass and inertia which results in high operating speed and low mechanical shock.
Normally, the total sequence from fault to clear may be accomplished in less than three cycles. Since energy dumped into a fault is proportional to time, the faster clearing action means less damage, lower contact erosion, longer maintenance free contact life, and maximum equipment protection. Thus it is desirable to employ vacuum interrupters.
A problem with the use of vacuum circuit interrupters is that if there is a loss of vacuum as by leakage of air through a crack caused by undue mechanical stresses, both the high strength of the vacuum dielectric and the rapid recovery characteristics are lost. The small electrode spacing will no longer be able to sustain the high voltages. Arcs and flashovers will occur. The white hot arc will burn the electrode and melt the envelope, and may even extend into and attack other parts of the interrupter assembly. Such leaks are not visible to an operator and failure has in the past been detected by complex measurements.
Desirability of measuring the pressure inside a vacuum interrupter to anticipate failure has long been recognized. In recent years pressure measuring systems for vacuum circuit interrupters have been developed. One such pressure measuring system is described in U.S. Pat. No. 2,864,968. The system employs a DC voltage applied between electrodes to cause electrons to be emitted. The electrons collide with any gas molecule within the envelope and positive ions will flow to the shield which is maintained at a negative potential. A measuring circuit is connected to the shield and measures the current. An increase in current indicates loss of vacuum. U.S. Pat. No. 3,263,162 describes a system which is connected between the shield and one electrode to provide a voltage therebetween and the current flow is then sensed. If there is a leak, ions are formed and the current flow indicates the loss of vacuum. The system includes a separate D.C. power supply and means providing a magnetic field for directing the ions in a helical path.
In power systems it is important to know whether a leak has occurred while the contacts are open or closed during operation of the circuit with which the interrupter is associated. If a leak is sensed while the contacts are open in a three phase ungrounded system, it is possible to remove and replace the vacuum circuit interrupter without danger of catastrophic arcing during removal. On the other hand, if the leak occurs when the contacts are closed in a grounded three phase system, the power must be turned off upstream of the current interrupter in order to be able to remove and replace the interrupter. If this is not done, an arc will be drawn and the equipment damaged when the interrupter is removed.